Because rotary regenerators for gas turbine engines are now being made of a ceramic material, they require seals formed from material that can provide sufficient coating wear life. To achieve this life, this material must be oxidation resistant at temperatures up to and exceeding about 1600.degree. F., and have a low coefficient of friction to minimize torque loads on the regenerator. U.S. Pat. No. 3,481,715 discloses a regenerator seal comprised of a surface layer of nickel oxide, calcium fluoride, and calcium oxide on a steel substrate. An intermediate layer comprising an alloy of nickel with aluminum or chromium may be added to improve the adhesion of the surface layer to the substrate.
A problem with the prior art regenerator seal is the reactive nature of its constituents, especially at temperatures greater than 1600.degree. F. Both nickel oxide and calcium fluoride tend to react with underlying metallic elements. For example, the chrome, aluminum, and iron in the substrate or intermediate layer tends to strip oxygen from the nickel oxide, while the calcium fluoride tends to react with aluminum to form aluminum fluoride gas phases. These reactions accelerate the oxidation of the metallic bond coating and metallic substrate, causing deterioration of the bonding interface, and swelling of the regenerator seal. This swelling may result in binding of the rotating regenerator core, which produces high torque loads and can result in core failure. Core binding may also produce high frictional shear loads at the coating-regenerator interface, which can lead to accelerated wear of the regenerator seal and core. The swelling, which occurs predominantly in the metallic bonding coating layer, can also produce high interfacial shear stresses at coating interfaces, and can lead to premature coating failure, particularly when accompanied by high frictional shear loads resulting from core binding.
One proposed solution to this problem has been to use a surface layer of zinc oxide, calcium fluoride, and tin oxide, with an intermediate coating of nickel oxide and calcium fluoride. However, even with these coatings the above-described problems at interfaces between metal alloys and nickel oxide and calcium fluoride are still encountered, particularly above 1600.degree. F. Additionally, zinc oxide and tin oxide have chemical compatibility problems of their own. In particular, zinc oxide and tin oxide will be stripped of their oxygen by metallic elements such as chrome, iron, and aluminum. Additionally, if elemental nickel is present, perhaps from reduction of nickel oxide in the underlying intermediate layer, tin and nickel may react to form Ni.sub.3 Sn.
Accordingly, there is a need for a coating for a regenerator seal that has a low coefficient of friction, good oxidation resistance, and has stable composition and bonding up to and exceeding temperatures of 1600.degree. F.